It is known in the art to store information or data in various kinds of memories. Generally, memory devices are known, which store information by electrical charges on capacitances, or by changes in resistance in an either volatile or non-volatile manner.
One example for a memory element is a polysilicon fuse. In order to write information on the fuse, typically a small conductive path of polysilicon is provided to be destroyed by an over current imposed to the path. Other techniques apply external laser light to dissect conductive portions of the fuse. A modified fuse, exhibits modified electrical properties, e.g. a larger electrical resistance, than the unmodified one. The modified state is also called the “written” state or the “blown” state, since a specific part of the fuse is destroyed. The written (or modified) state is usually associated with a logic value, for example a logic ‘1’. Accordingly, the unmodified state is assigned to logic ‘0’. However, for the present invention, these convention are only relevant to ease the understanding of the illustrative embodiments.
Once, the physical characteristics of a polyfuse are modified, the written fuse provides an increased electrical resistance, the value of which can be in an order of magnitude higher than the resistance of the unmodified fuse. Combinations of modified and unmodified non-volatile memory elements represent a specific information stored permanently on the memory elements.
Another new class of non-volatile memories is based on the use of materials having a programmable resistance. Memories based on these materials have the advantage that they can be better scaled down to smaller sizes than charge-based memories such as DRAM. The most prominent technologies are magneto-resistive random access memories (MRAM), phase change memory, the programmable metallisation cell (PMC), the RRAM, and molecular storage.
Generally, the storage elements of the above technologies are resistors with at least two non-volatile resistance states. A particular resistance state can be programmed by application of either a voltage, a current, or both. The above mentioned memory elements can be used as read-only memories or as rewritable memories. Magneto-resistive random access memories (MRAM) are non-volatile memory devices, wherein the information is stored by means of magnetic charge elements. This kind of memories uses material properties that changed their electrical resistance, when a magnetic field is applied. Data retrieval is a simple matter of detecting the relative resistance. Different mechanisms are known in the art. Other memories to which the invention relates are ferroelectric RAM (FRAM).
Still another example for a memory element being susceptible for the present invention is an EEPROM, in particular a flash EEPROM. These elements provide a floating gate, which is isolated from the control gate. The floating gate is charged or discharged by different mechanisms, such that the electrical properties of the device, in particular of the channel through the device are modified. The modification can be permanent, or temporary.
According to prior art reading mechanisms for the above memory elements, the information stored on a memory element is evaluated by means of a complex analogue circuitry. The value representing the stored information of the memory element is usually converted in either a current or a voltage difference. For resistive memory devices, the memory element is usually coupled to a current source or a voltage source, supplying a defined current or a voltage to the device such that the corresponding current or voltage arising on the element relates to its resistance. For memory elements based on charges stored on capacitors, there is usually a sensing amplifier or the like for determining whether there is a specific charge on the capacitor representing the stored information. The output voltage of the sense amplifier indicates the stored information, wherein usually the stored information is destroyed.
The so established voltage or current value, represents the stored value in itself or it is compared to a reference value, that is usually generated by use of a reference element of a predefined value. The voltage or current values relating to the reference element and the memory element are compared by means of a comparator. The result of the comparison indicates, whether the element is in a modified or unmodified state.
One drawback of the described prior art solution consists in the high power consumption phase while the currents or voltages are applied during the read out of the information contained in the memory element. High power consumption is disadvantageous particularly for mobile devices, i.e. low power applications. To overcome this problem, prior art solutions suggest to lower the supply voltage for further power savings. However, such a measure impairs analogue sensing sensitivity and robustness and impacts the yield and the reliability of the measuring devices severely. As for integrated circuitry, to maintain sufficient reliability of the electrical design behavior, designers must increase the dimensions of the integrated components to meet the matching requirements, which in turn, leads to a dramatic bit cell size overhead. The same problem arises from the ongoing downscaling of integrated devices entailing lower supply voltages.
U.S. Pat. No. 6,930,942 B2 discloses a method and an apparatus for measuring current for memory cell sensing purposes. The sensing circuit includes an amplifier, a capacitor, a current source circuit, a clocked comparator, and a clocked counter. The current source circuit operates responsive to an output of the comparator to supply or withdraw current to and from the capacitor during respective charging and discharging intervals. The count in the clocked counter results from periodic comparisons of the capacitor voltage with the reference voltage and is, therefore, related to the logic state of the memory cell.